Video: ATK-built LM7 “Truck Engine” Pumps Out More Than 500 HP

We’re living in a day and age where 500 horsepower on the street is far more common than it used to be. Thanks to advancements in technology and parts development, shops like ATK High Performance Engines out of Grand Prairie, TX are able to make a 500-plus horsepower, naturally-aspirated 383 cubic-inch engine look easy, with an OE block and cylinder head castings to boot! The engine in the video – an ATK LM7-LB-3 – started life as a humble 5.3-liter Gen III LM7 truck engine, making somewhere between 270 and 295 horsepower (depending on the year it was produced) before ATK put their spin on it.

Of that original engine, only the factory iron block and aluminum cylinder head castings are reused. ATK starts by opening up the bores by almost .125-inch to 3.903 inches. Then they drop in a forged 4340 steel crank from K1 Technologies with a 4.00-inch stroke. They then attach a set of 6.125-inch Scat I-beam rods to Wiseco forged pistons to complete the rotating assembly and get 383 cubic-inches of displacement. A hydraulic roller camshaft from Howards Cams runs the show, measuring .604-inch at both valves, with 218 degrees of duration at .050 on the intake and 224 degrees on the exhaust side. A 112-degree lobe separation angle makes for a solid, streetable powerband.

The top half of the engine is deceptively simple. Starting with Texas Speed & Performance’s CNC-ported cathedral-port OEM #706 heads, they add a 2.02-inch intake valve, and 1.60-inch exhaust valve with dual springs and titanium retainers and the original 1.7:1 rockers, but with the Straub Technologies trunnion upgrade kit installed. On this particular engine, an Edelbrock Performer RPM intake manifold and Quick Fuel Technology 750 double-pumper are used; but the long-block will also work with a stock or aftermarket EFI intake, 36 pound-per-hour injectors and some factory ECM tuning. As has been said before, and will undoubtedly be said again, it’s a great time to be a gearhead!